1. Field of Invention
The present invention relates to a wavelength-division multiplexing optical communication system for wavelength-division multiplexing (WDM) a plurality of optical signals having different wavelengths and transmitting a wavelength-division multiplexed signal via an optical fiber transmission line.
2. Description of the Related Art
As future multimedia networks are built, there will continue to be an increased demand for an optical communication system with a higher capacity necessary for building future multimedia networks. As the Internet, broadband ISDN (B-ISDN), and so forth, increasingly become more popular and as several-Mbps of information are handled for enjoying dynamic image communications at home, a terabit (Tbps=1,000 Gbps) transmission capacity of a trunk system will soon be required. A terabit-transmission capacity is orders of magnitude larger than a current communication capacity a telephone network, which is 64 kbps. Therefore, interest in time-division multiplexing (“TDM”), optical time-division multiplexing (“OTDM”), wavelength-division multiplexing (“WDM”), and so forth, as multiplexing technologies for realizing a mass communications capacity has also increased.
WDM technology makes use of a wide gain bandwidth of an Erbium-Doped Fiber Amplifier (“EDFA”) for amplifying an optical signal on an optical level, and promises to be a flexible means for performing a cross-connect or an add/drop operation on an optical level, or for realizing a light wave network. Because of the progress resulting from the study and development of the WDM technology, attempts at developing an optical fiber amplifier in a wavelength-multiplexing optical fiber amplifier based on the EDFA have also been actively made.
A wavelength-multiplexing optical fiber amplifier is a key component of a wavelength-division multiplexing communication system. The wavelength-multiplexing optical fiber amplifier normally amplifies a wavelength-multiplexed optical signal that has a plurality of wavelengths using a single-mode optical fiber on which a rare-earth ion, such as an erbium ion (Er3+), is doped. The most typical erbium-doped optical fiber amplifier has a wide gain bandwidth at 4 THz or more over approximately a 35 nm wavelength range from 1530 nm to 1565 nm. Amplification is made in one step by wavelength-multiplexing optical signals that have several tens to one hundred different wavelengths within this gain bandwidth.
A wavelength-multiplexing optical fiber amplifier, which is one of the key components of a wavelength-division multiplexing optical communication system, has the following problems which arise due to the simultaneous amplification of a plurality of wavelength-multiplexed optical signals having different wavelengths.
(1) A wide bandwidth characteristic needed for amplifying a multi-wavelength signal,
(2) A wavelength flatness of a gain over a wide input dynamic range,
(3) Controllability of an optical output of each channel,
(4) Loss compensation of a dispersion compensator, and
(5) Optical output control of fluctuations in the number of input channels.
In addition, the wavelength-multiplexing optical fiber amplifier needs to have a low noise characteristic and a high output characteristic (or a high efficiency characteristic when a pump light power is converted into a signal light power).
The wavelength flatness of a gain over a wide input dynamic range is a serious problem when a wavelength-multiplexing EDFA is used as an optical amplification repeater (in-line amplifier). Over a wide dynamic range, only one optical amplifier is sufficient even if losses caused in repeater periods are different.
Each wavelength-multiplexed channel, or each wavelength, must be received at a reception end while maintaining a good quality. To implement this reception, lower and upper limits must be determined for the output of each channel of an optical amplification repeater. The problem associated with the controllability of an optical output of each channel occurs because the optical amplification repeater is not capable of generating waveforms of a generation repeater and extracting its timing, and therefore noises are accumulated. The upper limit is determined in order to prevent a signal waveform from degrading due to non-linear effects such as self-phase modulation (“SPM”), cross-phase modulation (“XPM”), and four-wave mixing (“FWM”), which arise in a single-mode optical fiber, or a transmission line. The lower limit is determined in order to prevent a signal-to-noise ratio (“SNR”) from degrading due to amplified spontaneous emission (“ASE”) from an optical fiber amplifier. The optical output of each channel (each wavelength) of the optical fiber amplifier must be between the upper and lower limits.
The problem associated with loss compensation of a dispersion compensator arises due to distortion of a waveform when a signal is transmitted at a high transmission speed, such as 10 GHz. This is because dispersion of approximately 18 ps/nm/km arises in a transmission line with a light having a 1.55 μm wavelength, which exists in an amplification bandwidth of the EDFA, if the transmission line is a 1.3 μm zero-dispersion single-mode optical fiber (“SMF”). To overcome this problem, there is a method for providing each repeater with a (negative) dispersion inverse to that which occurred in a fiber between repeaters. In addition, an insertion loss of a dispersion compensator is compensated for with an optical fiber amplifier.
The problem associated with optical output control of fluctuations in the number of input channels is serious when a wavelength-multiplexing optical fiber amplifier is applied to a light wave network to perform a cross-connect, or an add/drop operation. That is, the number of channels input to the optical fiber amplifier varies during operations. However, the output of each channel must keep at a predetermined value.
To overcome problems associated with optical output control of fluctuations in the number of input channels, it is necessary to arrange an optical service channel for controlling the optical fiber amplifier, and to cope with a change of the number of channels using the service channel.